Fish are the main source of animal protein for about a quarter of the world�s population. The farming of the sea (sometimes known as 'aquaculture') provides a large number of jobs. However, the fishing industry can have negative impacts on the environment. Some fish stocks are being over-exploited at unsustainable levels, and other species (e.g. dolphins and seabirds) are affected by problems caused by fishing, such as discarded nets which can trap them underwater. Over-fishing can also affect the rest of the marine ecosystem by disturbing the natural balances that exist between different aquatic species. Pollution also has a significant impact on the aquatic environment. 70% of sea pollution has come from activities on the land, and some fish stocks are being damaged, especially shellfish.

Some of the policies which are suggested by Agenda 21 to make the fishing industry more sustainable include reducing the amount of fish which may be caught by each country. This is known as the 'quota.' Licensing fishing boats and banning fishing in parts of the sea which have been over-exploited in the past, should allow fish stocks to recover.